In golf balls commercially selling, there are solid golf balls such as two-piece golf ball, three-piece golf ball and the like, and thread wound golf balls. Recently, the solid golf balls, of which flight distance can be improved while maintaining soft and good shot feel at the time of hitting as good as the conventional thread wound golf ball, generally occupy the greater part of the golf ball market. Multi-piece golf balls represented by three-piece golf ball have good shot feel while maintaining excellent flight performance because they can vary hardness distribution, when compared with the two-piece golf ball.
As representative examples of multi-piece solid golf balls, there is a three-piece solid golf ball comprising two-layer core or two-layer cover. As multi-piece solid golf balls that can further vary hardness distribution, there is a four-piece solid golf ball comprising two-layer core and two-layer cover, three-layer core or three-layer cover and the like have been proposed (Japanese Patent Kokai Publication Nos. 266959/1997, 127818/1998, 127819/1998, 245873/2000, 17572/2001, 17575/2001 and the like).
In Japanese Patent Kokai Publication Nos. 266959/1997, 127818/1998 and 127819/1998, a four-piece solid golf ball which comprises a cover and a three-layer core composed of an inner layer, an intermediate layer and an outer layer is described. In Japanese Patent Kokai Publication No. 266959/1997, the inner layer is designed to have a Shore D hardness lower than that of the intermediate layer, the intermediate layer has a Shore D hardness of 45 to 65 and the outer layer is designed to have a Shore D hardness lower than that of the intermediate layer; in Japanese Patent Kokai Publication No. 127818/1998, the intermediate layer has a JIS-C hardness of 50 to 80 and the outer layer is designed to have a hardness higher than that of the intermediate layer; and in Japanese Patent Kokai Publication No. 127819/1998, the inner layer JIS-C hardness of 40 to 90, the intermediate layer is formed from thermoplastic resin and has a JIS-C hardness of 50 to 80, and the outer layer has a JIS-C hardness of not less than 65.
In Japanese Patent Kokai Publication Nos. 17572/2001 and 17575/2001, a four-piece solid golf ball which comprises a cover and a three-layer core composed of a core, an enclosing layer and an intermediate layer is described. In Japanese Patent Kokai Publication No. 17572/2001, the core is essentially formed from thermoplastic resin or thermoplastic elastomer and has a diameter of 3 to 18 mm and a Shore D hardness of 50 to 95, the enclosing layer is essentially formed from thermoplastic resin or thermoplastic elastomer (and has a Shore D hardness lower than that of the core by not less than 10); in Japanese Patent Kokai Publication No. 17575/2001, the core is essentially formed from thermoplastic resin or thermoplastic elastomer and has a diameter of 3 to 18 mm and a Shore D hardness of 15 to 50, the enclosing layer is essentially formed from thermoplastic resin or thermoplastic elastomer, and the enclosing layer and intermediate layer have a substantially equal Shore D hardness at a boundary therebetween. In the five golf balls, since the three-layer core is not designed to have a hardness distribution such that the outer portion is hard and the inner portion is soft, it is impossible to efficiently deform the golf ball and the launch angle is low, which reduces the flight distance.
In Japanese Patent Kokai Publication No. 245873/2000, a four-piece solid golf ball comprising a core, an intermediate layer, an outer layer and a cover is described. The core has a surface hardness in JIS-C hardness of 67 to 85, the intermediate layer hardness in JIS-C hardness is higher than the surface hardness of the core, and the outer layer hardness in JIS-C hardness is higher than the intermediate layer hardness. In the golf ball, the three-layer core has a hardness distribution such that the outer portion is hard and the inner portion is soft. However, since the hardness difference between the surface of the outer layer and the surface of the core is small, high launch angle and low spin amount are not sufficiently accomplished, which reduces the flight distance. In addition, the outer layer hardness is low, and the golf ball has dull and deep sound when putting. Since the cover is formed from hard material, spin performance at approach shot is not sufficient obtained.